A Third Generation Partnership Project (3GPP) Release 13 (Rel-13) study item focuses on “Licensed-Assisted Access” (LAA) operation in Long Term Evolution (LTE) networks, which allows the LTE networks and related equipment to operate in the unlicensed 5 GHz radio spectrum. More particularly, the unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum on a primary cell (PCell) and use carrier aggregation to benefit from additional transmission capacity provided by a secondary cell (SCell) operated in the unlicensed spectrum. To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called Listen-Before-Talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy—such sensing may be referred to as a “clear channel assessment.” Currently, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under the WI-FI name.
Because of the need to wait for a clear channel, the first slot in which the LAA SCell or LAA user equipment (UE) is permitted to transmit cannot be predicted in advance. This uncertainty makes it difficult to pre-compute the data payloads for transmission, because several transmit parameters are dependent on the slot number in which data is transmitted.
LTE uses orthogonal frequency-division multiplex (OFDM) in the downlink and Discrete Fourier transform (DFT)-spread OFDM (also referred to as single-carrier frequency division multiple access or SC-FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element or RE corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Rel-11 onwards, the above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in the above FIG. 3 are cell specific reference symbols (CRS) and are used to support multiple functions, including fine time and frequency synchronization and channel estimation for certain transmission modes.
The generation of the baseband transmit signal on the physical shared channels for either the Downlink (DL) or Uplink (UL) generally involve scrambling, modulation mapping, layer mapping, precoding, and RE mapping. The specific baseband chain for the UL PUSCH is shown in FIG. 4 as an example. For PUSCH scrambling, the initialization of the scrambling sequence generator at the start of each subframe is a function of the current slot number ns. This is also true for PDSCH scrambling on the DL.
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This compatibility should also include spectrum compatibility. Spectrum compatibility implies that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Rel-10 deployments, there likely will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to ensure that legacy carriers operate efficiently with respect to wide carriers, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to achieve this usage involves Carrier Aggregation (CA).
CA implies that an LTE Rel-10 terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5. A CA-capable UE is assigned a PCell that is always activated, and one or more SCells that may be activated and deactivated dynamically. The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink.
A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same, whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism allows an (enhanced) physical downlink control channel or (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH is also configured semi-statically.
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as busy or occupied, the transmission is essentially deferred until the channel is deemed to be idle or clear. When the range of several access points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. FIG. 6 provides a general illustration of the CCA operation, also referred to as a Listen-Before-Talk (LBT) mechanism.
One way to utilize unlicensed spectrum for LTE in a manner that is more reliable and more reflective of the need to coexist with other systems or devices sharing the unlicensed spectrum involves using the carrier(s) operating within licensed spectrum for the transmission of essential control signals and channels. FIG. 7 depicts an example of such an arrangement, where a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application we denote a secondary cell in unlicensed spectrum as licensed-assisted access secondary cell (LAA SCell).
Using the LTE DL transmission as a first non-limiting example, the downlink physical channel is scrambled as follows as described in 3GPP TS 36.211. For each codeword q, the block of bits b(q)(0), . . . , b(q)(Mbit(q)−1), where Mbit(q) is the number of bits in codeword q transmitted on the physical channel in one subframe, shall be scrambled prior to modulation, resulting in a block of scrambled bits {tilde over (b)}(q)(0), . . . , {tilde over (b)}(q)(Mbit(q)−1) according to{tilde over (b)}(q)(i)=(b(q)(i)+c(q)(i))mod2where the scrambling sequence c(q)(i) is given by clause 7.2 of TS 36.211. The scrambling sequence generator shall be initialized at the start of each subframe, where the initialization value of cinit depends on the transport channel type according to
      c    init    =      {                                                                      n                RNTI                            ·                              2                14                                      +                          q              ·                              2                13                                      +                                          ⌊                                                      n                    s                                    /                  2                                ⌋                            ·                              2                9                                      +                          N              ID              cell                                                            for            ⁢                                                  ⁢            PDSCH                                                                                          ⌊                                                      n                    s                                    /                  2                                ⌋                            ·                              2                9                                      +                          N              ID              MBSFN                                                            for            ⁢                                                  ⁢            PMCH                              where nRNTI corresponds to the RNTI associated with the PDSCH transmission as described in clause 7.1 3GPP TS 36.213.
It can be observed that the scrambling code sequence depends on the slot number ns. Other examples of slot-number dependent signal generation in the LTE system include Reference Symbols (RS), RS hopping, Physical Uplink Control Channel (PUCCH) cyclic shifts, etc.
Because of the LBT procedure, the first slot in which the LAA SCell or LAA UE is permitted to transmit cannot be predicted in advance. This fact in turn makes it difficult to pre-compute the data payload on physical channels, because several associated parameters such as scrambling, RS hopping, PUCCH cyclic shift etc., depend on the slot number in which data is transmitted. In other words, it is recognized herein that the same codeword will have to be continually re-scrambled for prospective transmission, for each slot that the LBT procedure is not successful.